Material Insights · Understanding the Material World

Combined heat and power: Science and thermodynamics

Why 82% efficient still flatters low-grade hot water

What this tab covers The thermodynamics: the grade (temperature or pressure) of the energy being recovered, energy versus exergy efficiency, and how harsh the waste stream is on equipment.

CHP turns 75-82% of fuel energy into useful electricity and heat, against 50-55% when power and heat are produced separately. Much of the recovered heat is low grade, so the gain in exergy terms is smaller than the headline suggests.

Grade of energy

Favourable

A gas engine CHP unit offers two heat grades: exhaust at 700-1,000 F (about 370-540 C), which can raise steam, and jacket water, oil and intercooler heat below 300 F (about 150 C), typically delivered as hot water at 180 F supply and 140 F return (about 82 C and 60 C). Absorption chillers turn this heat into cooling (trigeneration): single-effect units run on hot water at 200-240 F (about 93-116 C) or steam at about 15 psig (about 1 bar gauge), while double-effect units need about 350 F (about 177 C) water or steam near 115 psig (about 8 bar gauge), or exhaust above 750 F (about 400 C).12

Engine exhaust 700-1,000 F (about 370-540 C)1 Engine low-temperature heat below 300 F (about 150 C)1 Single-effect chiller drive 200-240 F hot water or about 15 psig steam2 Double-effect chiller drive about 350 F hot water or about 115 psig steam2

Exergy versus energy efficiency

Favourable

DOE puts reciprocating engine CHP at 75-82% overall (75-77% where steam is raised), with 27.7% (35 kW) to 40.6% (4.5 MW) as electricity and 38.4-54.0% as hot water. EPA compares this with 50-55% for separate grid power and on-site boilers. Hot water sent out at 82 C and returned at 60 C is worth only about 13% of its energy as work against 25 C surroundings (calculated), so on an exergy basis engine CHP delivers roughly 35-46% (calculated), still better than a typical fossil power plant at 36% (EPA) but far below the energy headline. Absorption chillers have coefficients of performance (cooling out per heat in) of 0.70-0.79 single-effect and 1.35-1.42 double-effect.123

Overall CHP efficiency (engines) 75-82%1 Separate heat and power 50-55%3 Engine CHP exergy efficiency about 35-46% (calculated)1 Absorption chiller COP 0.70-0.79 single, 1.35-1.42 double2

Media compatibility and corrosiveness

Mixed

On natural gas the heat streams are clean, and exhaust boilers rarely foul. Biogas, landfill gas and liquid fuels bring sulphur and siloxanes that attack engines and heat exchangers, so gas cleaning is part of the system. Rich-burn engines emit 10 g NOx per brake horsepower-hour or more before a three-way catalyst brings this to 0.1 or lower; lean-burn engines emit 0.5-1.0 g, and selective catalytic reduction (adding USD 130-375 per kW) removes 90-95%. Lithium bromide absorption chillers serve chilled water at 40 F (about 4 C) or above; colder duties use ammonia-water.12

Rich-burn engine-out NOx 10 g/bhp-hr or more1 Lean-burn engine-out NOx 0.5-1.0 g/bhp-hr1 SCR add-on cost USD 130-375/kW1

Variants compared1258914

VariantTypical sizeKey performance figureCost
Engine CHP (cogeneration)1 kW to 10 MW per engine; most systems under 5 MW75-82% overall efficiencyUSD 2,000-4,250/kW installed
Steam turbine CHPunder 100 kW to over 250 MWCan exceed 80% overallUSD 650-2,700/kW
Trigeneration with absorption chiller5-3,000 refrigeration tonsCOP 0.70-0.79 single, 1.35-1.42 double effectUSD 1,800-6,000 per ton single; 2,000-3,300 double
Adsorption chillerSmallSolid sorbent (silica gel) with water refrigerant; DOE gives no performance figuresNot quantified
Micro-CHP (e.g. ENE-FARM fuel cells)about 1 kW per home; engines from about 1 kW55% electrical, 87% total for SOFC Type S (company claim); 85% or more total across ENE-FARMBelow 2 million yen per unit by 2015, about a quarter of launch price (dated)

Sources

  1. Reciprocating Engines, CHP Technology Fact Sheet Series (DOE/EE-2764) US Department of Energy, 2023
  2. Absorption Chillers for CHP Systems, CHP Technology Fact Sheet Series (DOE/EE-1608) US Department of Energy, 2017
  3. CHP benefits (archived page, snapshot of 19 January 2025) US EPA Combined Heat and Power Partnership, 2025
  4. Steam Turbines, CHP Technology Fact Sheet Series (DOE/EE-2976) US Department of Energy, 2025
  5. Fuel cell systems: Ene-Farm Type S (company claims) Osaka Gas (Daigas Group), 2020
  6. Home fuel cell ENE-FARM and demand response (Fuel Cell Commercialization Conference of Japan, DR-ready study group, 25 November 2025) Ministry of Economy, Trade and Industry (METI) committee papers, 2025
  7. NEDO's activities on fuel cells, 23rd IPHE Steering Committee (ENE-FARM price and installations to March 2015) NEDO via International Partnership for Hydrogen and Fuel Cells in the Economy, 2015